I created my own diseases. I don't want Norovirus on my server. If I delete the file from the data folder, it is recreated when I start the server. Is there a way to stop this?

Yes there is.

In the config file located in */config/Diseases there is a setting called Loaded diseases on startup, make sure "Norovirus" is not in that list. If you do not want any viruses loaded by default you can set the value to an empty list []. It should look like this:

"Loaded diseases on startup": [],

Dont forget the "," after the brackets or it will break.